第89页 | Learning Python | 阅读 ‧ 电子书库

import and reload Usage Notes

For some reason, once people find out about running files using import and reload, many tend to focus on this alone and forget about other launch options that always run the current version of the code (e.g., icon clicks, IDLE menu options, and system command lines). This approach can quickly lead to confusion, though—you need to remember when you’ve imported to know if you can reload, you need to remember to use parentheses when you call reload (only), and you need to remember to use reload in the first place to get the current version of your code to run. Moreover, reloads aren’t transitive—reloading a module reloads that module only, not any modules it may import—so you sometimes have to reload multiple files.

Because of these complications (and others we’ll explore later, including the reload/from issue mentioned in a prior note in this chapter), it’s generally a good idea to avoid the temptation to launch by imports and reloads for now. The IDLE Run→Run Module menu option described in the next section, for example, provides a simpler and less error-prone way to run your files, and always runs the current version of your code. System shell command lines offer similar benefits. You don’t need to use reload if you use these techniques.

In addition, you may run into trouble if you use modules in unusual ways at this point in the book. For instance, if you want to import a module file that is stored in a directory other than the one you’re working in, you’ll have to skip ahead to Chapter 21 and learn about the module search path.

For now, if you must import, try to keep all your files in the directory you are working in to avoid complications.[8]

That said, imports and reloads have proven to be a popular testing technique in Python classes, and you may prefer using this approach too. As usual, though, if you find yourself running into a wall, stop running into a wall!

[7] Notice that import and from both list the name of the module file as simply myfile without its .py suffix. As you’ll learn in Part V, when Python looks for the actual file, it knows to include the suffix in its search procedure. Again, you must include the .py suffix in system shell command lines, but not in import statements.

[8] If you’re burning with curiosity, the short story is that Python searches for imported modules in every directory listed in sys.path—a Python list of directory name strings in the sys module, which is initialized from a PYTHONPATH environment variable, plus a set of standard directories. If you want to import from a directory other than the one you are working in, that directory must generally be listed in your PYTHONPATH setting. For more details, see Chapter 21.